The present invention relates to a limited slip differential and a manufacturing method for a limited slip differential.
Conventional limited slip differentials for vehicles include torque sensing limited slip differentials for limiting a differential motion in response to torque reaction force generated in a driving system. A typical torque sensing limited slip differential is provided with a ring gear and a sun gear which are placed coaxially, planetary gears which are engaged with these gears, and a planetary carrier for supporting the planetary gears in such a manner that the planetary gears rotate on their axes and revolve while making contact with and sliding against the surface of the teeth of the planetary gears. This limited slip differential allows for a differential motion between the two output axes on the basis of the rotation and the revolution of the planetary gears and limits the differential motion on the basis of the thrust friction force resulting from the rotation between the gears, which are engaged with each other and the frictional force between the surface of the teeth of the planetary gears and the planetary carrier which make contact with and slide against each other.
In the above described configuration, in which the surfaces of the teeth of the planetary gears make contact with and slide against the planetary carrier, the state of lubrication between the surfaces which slide against each other is extremely important. That is to say, lack of lubricant supplied between the surfaces which slide against each other may increase vibration and cause abnormal friction between the surfaces which slide against each other, which in turn causes seizing. Taking this into consideration, Japanese Laid-Open Patent Publication No. 2004-324736, for example, discloses a method for creating a plurality of irregular recesses on the surfaces of the teeth of the planetary gears. As a result, the state of lubrication between the surfaces which slide against each other is improved, so that the wear resistance and durability increase.
In recent years, a high level of quietness has been demanded for vehicles. Therefore, it is important in limited slip differentials to optimize the state of lubrication between the surfaces which slide against each other, from the point of view of improving the quietness, in addition to the wear resistance and durability.
In order to improve the quietness by reducing vibration in the limited slip differential, it is effective to make the coefficient of friction-sliding velocity characteristics (μ−v characteristics) close to a positive gradient, that is to say, make the characteristics such that the coefficient of friction increases as the sliding velocity increases. Increase in the thickness of the film formed on the surfaces which slide against each other, that is to say, increase in the thickness of the lubricant film, promotes the shift from boundary lubrication to mixed lubrication, which shift is caused as the relative velocity between the planetary gears and the planetary carrier, or the sliding velocity, increases. In this case, a solid friction component is the main constituent of boundary lubrication, and the fluid friction component increases in mixed lubrication. Meanwhile, excessive restriction of increase in the thickness of the above described film increases reduction in the solid friction component as the sliding velocity increases. That is to say, the frictional force between the surfaces which slide against each other decreases and the vibration of the limited slip differential increases in either case. Accordingly, it is particularly important to improve the state of lubrication between the surfaces which slide against each other in an increasing phase of the sliding velocity in order to further improve the quietness.